This invention relates generally to the fabrication of superconducting materials and is particularly directed to the magnetic orientation of the crystals of a superconducting polycrystalline metal oxide material in a preferred direction for enhanced current carrying capacity.
Certain metals, alloys, and chemical compounds known as superconductors exhibit zero electrical resistivity and complete diamagnetism at very low temperatures and magnetic fields. The transition of a metal from normal electrical conducting properties to superconducting properties depends primarily on (1) the temperature and (2) the magnetic field at the surface of the metal. The superconductive state of the metal exists for temperatures less than its characteristic critical temperature, T.sub.c. The most practical superconducting materials exhibit very low critical temperatures, i.e., on the order of 4- 23 K. However, recent developments have produced new superconducting materials, based on oxides, having critical temperatures on the order of 100 K.
Superconductors also exhibit a characteristic critical electric current density, J.sub.c, measured in amps/cm.sup.2. By increasing the current density in a superconducting material to its J.sub.c characteristic value, it can be driven into a normal conducting state. Thus, the current density at which this transition occurs is termed the conductive material's critical current density. It is of course desirable for a superconductor to have a high critical current density to allow it to conduct large currents while remaining superconductive.
In practical Type II superconductors efforts to increase the critical current density have involved the incorporation of microstructural defects in the material. A magnetic field applied to the superconductor penetrates the material in the form of small bundles, or vortices, of magnetic flux which can move about within the material as it conducts current. Movement of these magnetic vortices is a dissipative process characterized by resistive heating and thus represents a limitation in the material's current carrying capacity. Incorporation of the aforementioned microstructural defects within the material prevents the magnetic vortices from moving within the superconductor in response to current flowing therein. By preventing movement of these magnetic vortices, the material's critical current density can be substantially increased before the material assumes normal, non-superconducting current conducting operation.
The class of metal oxide superconductors unfortunately exhibit very small critical current densities. For example, these materials typically have a critical current density in the presence of magnetic fields on the order of 5-10 amps/cm.sup.2, at liquid nitrogen temperatures, while other practical superconductors typically are capable of supporting current densities on the order of 10,000-100,000 amps/cm.sup.2. As a result, these metal oxide materials have hereto fore been of limited use in superconducting applications. Recent investigations have indicated that low critical electric current density values are not an intrinsic problem with metal oxide superconductors, since single crystal, thin film metal oxide superconducting materials have been produced which are capable of supporting very large current densities.
The present invention addresses the aforementioned limitations of the prior art in that it contemplates the orientation of the crystal axes of a metal oxide superconducting material in the form of a powder comprised of single crystal particles along preferential directions for increasing the critical current density of the superconductor. In one embodiment, the crystal particles are aligned by means of a magnetic field applied while the material is in a superconducting state, while another embodiment involves the substitution of rare-earth or other magnetic ions in the metal oxide superconducting material and the application of an aligning magnetic field while in a normal, non-superconducting state.